Single track with bundle should be selected to define axis
Longwinded and rambling answer:
The scene has to be solved first.
Then the slected tracker has to be part of the ones used for solving the scene.
A bundled track is one that has a reprojected point.

Bundles makes sense after solving the movie clip, and it works in the following way: the solved position of each track gets projected back to the movie clip and displayed as a small point. The color of the point depends on the distance between the projected coordinate and the original coordinate: if they are close enough, the point is green, otherwise it'll be red. This helps to find tracks which weren't solved nicely and need to be tweaked.

(What does &quot;three tracks with bundles are needed to orient the floor&quot; mean?)
Reprojected points can be viewed as an overlay by enabling Marker Display &gt; 3D markers

A green reprojection dot means that the tracker and reprojection have an acceptable error, a red dot means that the error is too high.

So to set an axis or origin or scale use a tracking point with a green dot.
Then the origin has to be set. To do that then you select a single tracker and that will be the 0, 0, 0 coordinate for the reconstructed scene.
Once you have the origin. Select a second tracker that is in the direction of the axis you want to determine. Then Click on the Set Axis option. The axis is thus determined using the selected tracker and the origin.

Note that you can bypass all of this steps ans manually orient and scale the scene any time you want later.
read: Correct tracked camera orientation